It is proposed to complete the development of newly devised electromagnetic methods for measurement of average and phasic volume rate of blood flow, blood velocity and vascular diameters. One method utilizes a magnet external to the animal's body so that its field traverses the blood vessel under study inducing an e.m.f. in the blood stream which constitutes the flow signal. The induced flow signal is picked up by means of an ultra-miniature intravascular probe which is inserted into the vessel under study much as is a guide wire via an angiographic catheter (the probe is a loop made of an insulated spring wire 0.1 mm in diameter). The loop probe can be inserted into small branches of the aorta via angiographic catheters as small as #4 French. The same transducer can simultaneously detect even minute variations in phasic and average arterial diameter. In another method, the magnetic field allowing absolute measurement of vascular diameter is supplied by passing a current through the loop probe itself while the e.m.f. induced in another wire loop of the same probe by this field is measured as a function of the vessel diameter. It is proposed to develop such transducers to a high degree of perfection and to sizes permitting measurements in arteries and veins down to 1 mm in diameter, and to develop appropriate methods of conveying the sensing element to the appropriate blood vessel, assuring optimal orientation of the sensor relative to the vessel and to the magnetic field. This research will involve exploration of all of the important blood vessels supplying the vital organs (such as renal, mesenteric, internal carotid and coronary arteries) and development of the simplest possible techniques of recording such regional blood flows and vascular diameters in anesthetized animals. Improvement will be made in the design of the flow and diameter sensors as well as in the electronic channels for amplification of the flow and diameter signals and for the current supply to the magnet.